The fabrication of electronic microcircuits requires the use of substrates, heatsinks, electrodes, leads, connectors, packaging structures and other components capable of dissipating the heat generated by the active parts of the microcircuit or by the soldering, brazing or glass-sealing process. Moreover, those components that are in direct contact with the active microcircuit sections must have a thermal expansivity compatible with gallium arsenide, silicon, gallium-arsenide or any material used in the fabrication of the microcircuit.
Materials such as copper, silver, gold and aluminum which have high thermal conductivities for efficient heat dissipation also tend to have thermal expansivities much higher than materials such as gallium arsenide alumina or silicon which are used in the fabrication of microcircuit elements or their enclosures.
As disclosed in U.S. Pat. No. 4,083,719 Arakawa et al., it has been found convenient to use composites of copper and lighter materials such as carbon fibers in the manufacture of heatsinks, substrates and other heat-dissipating elements of microcircuits. The proportions of the carbon and copper in the composites are designed to match the thermal expansivity of the material used in the fabrication of the active circuit component.
The thermal expansivity of a material is defined as the ratio of the change in length per degree Celsius to the length at 0.degree. C. The coefficient of thermal expansion (CTE) is usually given as an average value over a range of temperatures. Metals used in electrical conductors such as aluminum, copper, silver and gold have a low electrical resistivity and exhibit high thermal conductivities. Thermal conductivity is usually measured in W/m*K. Copper has a K value of 386 and silver has a value of 419. However, these metals have a thermal expansivity in excess of 15.times.10.sup.-6.degree. C..sup.-1. Thus, while material of high electrical and thermal conductivity are favored in the fabrication of heat-dissipating electric elements, they must be blended with conductive materials exhibiting a much lower average thermal expansivity in order to create a composite whose thermal expansivity characteristic comes as close as practical to that of gallium arsenide, silicon and other microchip materials. A variety of carbon products having thermal expansivities in the range of 1 to 2.times.10.sup.-6.degree. C..sup.-1 and thermal conductivity values in the range 100 to 600 W/m*K are considered excellent materials for controlling both thermal conductivity and thermal expansivity in the composites.
Whereas, copper, aluminum and silver have specific gravities of about 9 g/cm.sup.3 and melting points less than 1,100.degree. C., carbon materials have specific gravities in the range 1.5 to 2.5 g/cm.sup.3 and do not melt, but sublime at temperatures above 3,000.degree. C. Due to the large differences in the specific gravities and melting points, lack of mutual solubility and lack of wetting of the copper on carbon, it is difficult to form carbon/copper composites by conventional melting methods.
The current invention results from an attempt to devise a simple and practical process for the manufacture of such heat-dissipating components carbides by using powder metallurgy or roll-forming techniques.
In addition, as disclosed in U.S. Pat. No. 4,680,618 Kuroda et al., it has been found convenient to use composites of copper and other denser metals such as tungsten or molybdenum in the fabrication of heatsinks, substrates and other heat-dissipating elements of microcircuits. The proportions of the metals in the composite are designed to match the CTE of the material used in the fabrication of the active circuit component.
As described above, metal used in electrical conductors having a low electrical resistivity such as aluminum, copper, silver and gold also exhibit high coefficients of thermal conductivity. However, these metals also exhibit high average CTEs. Therefore, tungsten and molybdenum with average CTE of 4.6.times.10.sup.-6 /.degree. C. and 6.times.10.sup.-6 /.degree. C. and coefficient of thermal conductivity of 160 and 146 respectively are favored as conductive metal candidates for blending.
However, while copper, aluminum, and silver have specific gravities of less than 9, and melting point of less than 1,100.degree. C., tungsten and molybdenum have specific gravities of 19.3 and 10.2, and melting points of 3,370.degree. C. and 2,630.degree. C. respectively.
Due to the large differences in the specific gravities and melting-points, and lack of mutual solubility of metals such as copper and tungsten, for example, it is difficult to form composites of those two metals that exhibit a reliable degree of homogeneity using conventional melting processes.
As disclosed in U.S. Pat. No. 5,086,333 Osada et al., it has been found more practical to press and sinter a powder of the most dense materials, e.g., tungsten, to form a porous compact, then to infiltrate the compact with molten copper or another lighter material. A slab of the resulting material can then be cut and machined to form heatsinks, connectors, substrates and other heat-dissipating elements.
The heat-dissipating base upon which micro-chips are mounted must also be attached to packaging or frame member usually made of ceramic or other material having a different CTE than the semiconductor material of the micro-chip and of the heat-dissipating base. Thermal stress between the heat-dissipating base and the frame member may cause cracks or camber after joining operation in the latter stage. The problem has been addressed in the prior art by using a intermediate heat-dissipating member whose composition and CTE continuously vary from one contact to the other as disclosed in U.S. Pat. No. 3,097,329 Siemens.
Another approach disclosed in U.S. Pat. No. 4,427,993 Fichot et al. consists of embedding a lattice of CTE-modifying material into one of the contact surfaces of the heat-dissipating element.
Both of these approaches are, complex and onerous and do not allow a precise control of the CTEs at one or both interfacing areas of the heat-dissipating member.
The costs of metals such as tungsten and molybdenum are relatively high compared to the costs of copper, aluminum and other more abundant metals. Heat-dissipating components made of composites in which a costly metal such as tungsten is used for CTE-matching purpose tend to be expensive. As the power ratings of micro-electronic modules increase, bigger heat-dissipating substrates are required. The cost associated with the substrates being dictated by the cost of their base metals remains inflexibly high while the costs of the microcircuit can be controlled and even reduced through the use of new technological improvements. The problem of substrate-related costs is particularly acute in microwave power devices where large and elaborate heat-sinks must be used. In many cases, the costs of the heat-dissipating component represent a large percentage of the total device. Accordingly, there is a need for a more economical way to construct heat-dissipating substrates for high-power micro-electric modules.
The instant invention results from an attempt to devise a simpler, more practical and more economical process to manufacture such heat-dissipating components using powder and other metallurgy techniques. The invention is based in part on the techniques and processes disclosed in the parent application, Ser. No. 08/064,255 which issued as U.S. Pat. No. 5,413,751 dated May 9, 1995, which application and patent are hereby incorporated in this Specification by reference.